Maturation and differentiatiin of mononuclear phagocytes is accompanied by changes in the biosynthesis and secretion of the major histocompatibility complex class III linked proteins C2, C4 and factor B. These changes have been shown to be species and tissue specific. The availability of cDNA probes specific for the class III proteins will enable studies of the regulation of synthesis of these proteins at a molecular level and provide a basis for evaluating the response to inflammation from different species.